This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-095527, filed Mar. 30, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an image formation method and apparatus, and more specifically to an image formation method and apparatus which use a liquid developer and recover solvent vapor which is generated in evaporation of the liquid developer.
An image formation apparatus using a liquid developer has advantages over a dry type of image formation apparatus and its values have recently been recognized again. For example, the main advantages of the wet type of image formation apparatus over the dry type are: high image quality can be achieved since very small toner particles of submicron dimensions can be used; economy can be effected because even a small amount of toner is enough to obtain good image density and moreover the same level of texture as in printing (e.g., offset printing) can be provided; and energy saving can be effected since the toner can be fixed onto a paper at a relatively low temperature.
The conventional wet developing image formation technology involves some essential problems and have therefore allowed the dry developing technology to occupy the dominant position over a long time.
The problems with the wet developing technology are:
(1) To develop an electrostatic latent image, a high-resistivity or insulating petroleum solvent must be used as the carrier solvent in the liquid developer. There is the possibility that bad odor resulting from volatilization of the solvent and the volatilized solvent may produce allergic reactions in the human body.
(2) Not only toner but also a large quantity of solvent adheres to the electrostatic latent image in the development process, which requires a step of removing the excess solvent after development and a step of removing the toner suspended in the solvent. It is further required to collect and remove the solvent vapor volatilized in the air in all the steps including transfer and fixing ones.
(3) An environmental problem may arise from adhesion of the solvent to paper. In field transfer of the liquid developer, toner particles, or electrically charged particles are moved in the solvent by electrophoresis and then transferred to a paper. In the field transfer, therefore, there must exist a predetermined quantity of solvent between the latent image carrier and the paper. As a result, a large quantity of solvent will adhere to the paper after transfer. Part of the solvent volatilizes by heat in the fixing process and then escapes from the image formation apparatus. The resulting odor and vapor will adversely affect the human body. Also, there is the possibility that the paper ejected out of the apparatus after fixing may contain a large quantity of solvent. This may adversely affect the user who handles the paper.
To solve such problems, a method has been proposed by which temporal transfer is made from a latent image carrier to an intermediate transfer medium and transfer to paper is then made. In U.S. Pat. Nos. 5,148,222, 5,166,734, and 5,208,637 are disclosed methods that transfer an image from the latent image carrier to the intermediate transfer medium and then transfer the image to a paper by pressure (and heat).
Those proposed techniques do not suffer from the problems associated with the field transfer since transfer from the intermediate transfer medium to the paper is made by heat and pressure. In addition, the solvent adhered to the intermediate transfer medium can be vaporized or sucked by heating or air suction prior to the pressure transfer to paper. In this manner, the amount of solvent adhered to the paper can be reduced. Moreover, the pressure transfer to paper can be made with no solvent involved.
However, as the liquid developer, use is normally made of one which is composed of a petroleum insulating solvent and electrically charged particles (toner particles) dispersed in the solvent. When such a solvent is used, organic solvent vapor is produced by natural volatilization in all places in the image formation apparatus where the liquid developer is present. The production of the solvent vapor by volatilization cannot be prevented perfectly. Trying to remove the solvent in the liquid developer from the photosensitive drum, intermediate transfer roller, transfer roller, or paper by thermal vaporization will cause a substantial quantity of solvent vapor to be produced in the apparatus, so that the apparatus is filled inside with the solvent vapor.
In this case, from the aforementioned reasons it is not desirable to discharge air in the apparatus to outside. Accordingly, recovery of the solvent vapor is made within the apparatus. For example, in Japanese Patent Application KOKAI Publication No. 48-82835, the solvent vapor produced around the fixing section is sucked and liquefied for recovery. Even with the liquefaction by cooling, however, the vapor-recovering efficiency is poor since the density of the solvent vapor cannot be lowered to less than its saturated vapor pressure at a cooling temperature. Although the solvent vapor density has been lowered to the saturated vapor pressure, it is not desirable to discharge the solvent vapor from the apparatus as it is.
Even if solvent vapor recovery equipment, such as cooling liquefaction equipment, is installed, its recovering capacity will be exceeded in the event that solvent vapor is produced continuously as in continuous operation for a long time. In such a case, the solvent vapor density may increase.
When the interior of the apparatus has to be exposed to outside as in the case of maintenance or paper jam, high-density solvent vapor will be discharged. To prevent this, the solvent must be recovered in a more efficient manner.
Accordingly, it is an object of the present invention to provide an image formation method and apparatus capable of efficiently suppressing the discharge of high-density solvent vapor from the apparatus.
According to one aspect of the present invention, there is provided an image formation apparatus comprising: a latent image carrier which is rotatable; a developing unit which supplies a liquid developer to the latent image carrier and develops a latent image formed on the latent image carrier, the liquid developer containing solvent and toner particles; a collecting unit which collects solvent vapor of the liquid developer; a separating unit which separates the solvent vapor from air collected by the collecting unit; a sensor which measures a density of solvent vapor in the separating unit; and a control unit which controls a rotational speed of the latent image carrier in accordance with the density of solvent vapor measured by the sensor.
According to another aspect of the present invention, there is provided an image formation apparatus comprising: a latent image carrier which is rotatable; a developing unit which supplies a liquid developer to the latent image carrier and develops a latent image formed on the latent image carrier, the liquid developer containing solvent and toner particles; a collecting unit which collects solvent vapor of the liquid developer; a separating unit which separates the solvent vapor from air collected by the collecting unit; and a control unit which controls a rotational speed of the latent image carrier on the basis of a number of pages to be printed per unit time.
According to still another aspect of the present invention, there is provided an image formation apparatus comprising: a latent image carrier which is rotatable; a developing unit which supplies a liquid developer to the latent image carrier and develops a latent image formed on the latent image carrier, the liquid developer containing solvent and toner particles; a collecting unit which collects solvent vapor of the liquid developer; first and second separating units which separate the solvent vapor from air collected by the collecting unit, the first and second separating units differing in their amount of solvent vapor to be separated per unit time; a control unit which selects one of the first and second separating units in accordance with variation of an amount of solvent vapor in one of the first and second separating units; and a restoration unit which restores a vapor-separating capacity of one of the first and second separating units, which separates larger amount of solvent vapor per unit time than the other.
According to still another aspect of the present invention, there is provided an image formation apparatus comprising: a latent image carrier which is rotatable; a developing unit which supplies a liquid developer to the latent image carrier and develops a latent image formed on the latent image carrier, the liquid developer containing solvent and toner particles; a collecting unit which collects solvent vapor of the liquid developer; first and second separating units which separate the solvent vapor from air collected by the collecting unit, the first and second separating units differing in their amount of solvent vapor to be separated per unit time; and a control unit which selects one of the first and second separating units on the basis of a number of pages to be printed per unit time.